A wireless local area network, such as a wireless LAN conforming to IEEE802.11a/b/g, refers to interconnecting computer devices by using wireless communication techniques, and constructing a network system capable of inter-communication and resource sharing.
In a wireless LAN environment, a typical network topology structure is called an extended service set. As shown in FIG. 1, an extended service set system consists of one or more basic service sets and a distributed system for connecting these basic service sets. Each basic service set includes a wireless access point device. The extended service set is distinguished by an extended service set identifier, and the wireless access point is distinguished by a basic service set identifier, i.e. an access point identifier. By scanning channels, the station obtains the information of the extended service sets and the basic service sets in the wireless LAN environment where the station locates, and chooses and joins a suitable basic service set.
When a station needs to be joined into a wireless LAN, the normal communication can only be started by connecting with the access point. The station at a certain time can only establish connection with one access point. If the station needs to change certain attributes of the connection while keeping the current connection, it may start another connection process.
After the station establishes connection with one access point, it checks whether there is an access point with better signal quality through scanning the channels, if such an access point exists, the station will establish connection with the new access point through a re-connecting process and cut off the connection with the old access point. This process is called handoff.
The current wireless LAN protocols do not prescribe the conditions for a station to perform handoff. Usually, it is realized by using the method of switching windows, i.e., when the station finds that the signal quality of a new access point is greater than that of the old access point by a predetermined handoff window value, the handoff will be performed. However, the violent variation of the wireless environment and unsuitably set handoff window value will lead to frequent handoff of the station between two access points with similar signal quality.